Okay, just to do it from basics - even if I don't particular like the combat mechanics as they are (I do love the wound system though). 5th edition did loose a lot of complexity and detail from earlier - some of it probably for the better - and we are missing rules for a number of circumstances that often arise during physical conflicts. Without making assumptions on everybodys behalf I do think that a fair share of us feel we need more on this and that a future book hopefully and expectedly will address it.
The basics:
Attack Total is Dex + Ability + Weapon + Die
Defense Total is Qik + Ability + Weapon + Die
I do not know if others follow the same practice but I tend to call the fixed elements, which is regularly anything but the stress die, for the respective Score (Characteristic + Ability + Weapon).
There are presumably endless ways to address the issue of surprise, so bear with me if I only list a few here.
You could:[i]*Ignore it. Keep it simple and don't change anything from a regular round of skirmish. Describe the carnage but dont get caught in any rule adjustment. Some feel it is easier to tell the story with less distractions from rules.
*You could grant automatic initiative to the attacker and disallow the defender an attack so that the defender can only defend for the first round.
*You could grant a bonus to the attackers Attack Total;
[list]a. either as a fixed bonus,
b. as a varying bonus (e.g. a 'Stealth' Advantage decided by opposed rolls prior to the ambush),
c. or as letting him add his Ability twice (but without the Fatigue loss)
*You could give a penalty to the defenders Defence Total;
a. either as a fixed penalty,
b. as a varying penalty,
c. or by disallowing him all or parts of his regular Defense Total (The Qik, the Ability, the Weapon, the Die roll, or a combo of these)
*You could entirely change the defenders Defence Total with a generic Ease Factor.
*You could add a considerable amount of extra botch die in case of a '0'. Particularly on the defenders roll, but an ambush can certainly also be a risky part on the attacker - even if you have the upper hand it is harder to predict people's moves when they are startled.
*Or you could just do it as some animals do (as Fruny pointed out that the HoH:MC mentions - he is the one with the bunny, alright!) and grant automatic initiative and +3 to the Attack Total the first round. And then perhaps ignore why natural predators can't catch their natural prey.[/i][/list:u]
One problem is that many of these things hinges on more general concepts which aren't in the combat system right now. But in the end this is most of all a challenge considering what stories you'd like to tell? Tales of grining taunting swashbucklers who always gets to snatch their swords out in time or is it about grim streets where every muddy shadowy corner might hide a dagger intended for your back?
Those where the options I could think of just now. From the above suggestions on how to rule it I know what I'd prefer and what I wouldn't...